Schottky diode is an important power device featuring a metal-semiconductor junction to create a schottky barrier for the purpose of rectification for a long time. With the characteristics of high switching speed and rectification, the schottky diode has been widely applied to high speed switching power devices, digital computers, and output regulators.
However, the schottky diodes fabricated by using semiconductor structures in present has the disadvantages of low reverse breakdown voltage and large reverse leakage current. In addition, the reverse current increases with increasing temperature to result in the problem of thermal instability. Thus, schottky diode should be operated under the reverse bias much smaller than the rated value, which brings some additional limitations to the application of schottky diode.
In view of the above mentioned problems, various improvements of semiconductor structures have been developed to enhance reverse breakdown voltage and reduce leakage current, and a commonly-used improvement is to form a boron-implanted termination (BIT), which is described below. FIG. 1 shows a cross section view of a conventional semiconductor structure. As shown, the semiconductor structure 1 includes a semiconductor substrate 11, an epitaxial layer 12, an ion implantation layer 13, an active region dielectric layer 14, a polysilicon layer 15, a termination region dielectric layer 16, a schottky barrier layer 17, and a metal electrode layer 18.
The epitaxial layer 12 is formed on the semiconductor substrate 11 and has a plurality of active region trenches 121 (only one of them is labeled) and a termination region 122. These active region trenches 121 are implanted with boron ions so as to form ion implantation layer 13 therein. In addition, the termination region 122 is also implanted with boron ions to form a boron ion implantation layer 1221 therein.
The active region dielectric layer 14 is formed on the inner wall and the bottom of these active region trenches 121, and the polysilicon layer 15 is formed on the active region dielectric layer 14. The termination region dielectric layer 16 includes a TEOS layer 161 and a BPSG layer 162. The schottky barrier layer 17 is formed on the epitaxial layer 12 and the termination region dielectric layer 16, and the metal electrode layer 18 is formed on the schottky barrier layer 17.
Since the impurity concentration of the semiconductor substrate 11 is greater than the epitaxial layer 12, the semiconductor substrate 11 is labeled as N+ and the epitaxial layer is labeled as N−.
Please refer to both FIG. 1 and FIG. 2, wherein FIG. 2 is a schematic view showing a simulation of the convention semiconductor structure. As shown in FIG. 2, the potential line inside the semiconductor structure 1 has a lower voltage level, that is the potential line closer the active region trench 121 and the termination region 122 has a lower voltage level. The potential lines show a contour smoothens longitudinally and converges at the right side of the semiconductor structure 1. It is understood that such structure may result in larger reverse leakage current under high reverse bias voltage such that the ability to withstand reverse breakdown voltage would be reduce. In addition, it is understood according to FIG. 2 that the termination region 122 formed by the implantation of boron ions needs a significant area for gradually lowering the electric field aggregated at the boundary of the active region to smoothen the voltage drop. There should be some room for the improvement of conventional structure to reduce reverse leakage current and enhance reverse breakdown voltage.
The parameters used in the simulation of FIG. 2 are, the depth of these active region trenches 121 is 2.4 μm, the impurity concentration of boron ion implantation within the ion implantation layer 13 is 4e2, the impurity concentration of boron ion implantation within the ion implantation layer 1221 in the termination region 122 is 5e4, and the thickness of termination region dielectric layer 16 is 7500 Å.
Accordingly, it is believed that people skilled in the art would notice that there exists a problem of high reverse leakage current and low reverse breakdown voltage of the conventional semiconductor structures because the ability to withstand the reverse breakdown voltage of the semiconductor structure would be reduced by the large reverse leakage current under high reverse bias voltage resulted from the implantation of P-type ions in the termination region.